The present invention is situated in the general field of optoelectronics, and relates to more precisely an optical multiplexer/demultiplexer.
The present invention relates to a multiplexer/demultiplexer having at least three waveguides, each pair of waveguides constituting distinct grating-assisted couplers so as to couple respectively at least two predetermined wavelengths.
The multiplexer/demultiplexer according to the invention can be used in an application for multiplexing and/or demultiplexing at least three wavelengths propagating in three different transmission windows whatever the numerical values of the latter.
Such a multiplexer/demultiplexer can also be used in an optical filtering application, the first coupler filtering a given wavelength and the second coupler, a so-called dummy coupler, serving as a rejection exit for another wavelength, close to the first, so as to refine the spectral response (or transfer function) of the first coupler.
The present invention particularly finds an application in the field of optical fibre distribution networks with direct access at the subscriber. In fact, in the field of optical telecommunications, the concept of FTTH (Fibre to the Home) has become an essential point in development for operators wishing to meet the ever increasing requirements of their customers.
Such distribution networks are already widely used and principally utilise optical fibres in which optical signals propagate in different transmission windows.
The optical signals are received and transmitted, and multiplexed and demultiplexed by optical modules.
Generally, the most usual case is to use two transmission windows, a first 1.3 xcexcm window for voice communications, and another 1.5 xcexcm window for video distribution.
FIG. 1 is a schematic diagram illustrating the principle of optical telecommunication by FTTH.
An Optical Line Terminal (OLT) provides communication between the different customers through an optical fibre distributor.
Each customer is equipped with an Optical Network Unit (ONU).
The two optical modules, the OLT and the ONU, are advantageously identical through their design. Laser diodes (LD) are used for sending an optical signal at a given wavelength, such as 1.3 xcexcm or 1.5 xcexcm, and photodiodes (PD) allow reception of the said optical signals.
The different wavelengths are multiplexed or demultiplexed in two steps. First of all, a filter separates the two wavelengths used and transmitted by optical fibres, and then a coupler separates the inputs and outputs of one and the same transmission window.
In the example illustrated, the 1.3 xcexcm window is used as an uplink channel and downlink channel for a so-called half-duplex voice communication, that is to say interference can occur between the signals propagating from the OLT to the ONU, and from the ONU to the OLT, the 1.5 xcexcm window being reserved for video distribution in a downlink channel only.
There exist other embodiments for obtaining a full-duplex voice communication, that is to say one without interference, using for example the 1.3 xcexcm window for the uplink channel and the 1.5 xcexcm window for the downlink channel. This embodiment must however abandon video distribution.
The present invention seeks to produce an optical transmitter which allows full-duplex voice communication, that is to say one on two different wavelengths for the uplink and downlink channels, whilst maintaining downlink video distribution.
To this end, the invention proposes to use a first 1.3 xcexcm transmission window allowing simultaneous bidirectional communication on two different wavelengths, at 1.3xe2x88x92 xcexcm and 1.3+ xcexcm, and another high transmission rate 1.5 xcexcm transmission window for video distribution simultaneous with voice communication.
Up to the present time, the 1.3 xcexcm and 1.5 xcexcm wavelengths were, in general, separated by a filter on two waveguides. The filtering function could be provided either by hybrid components provided with an adapted dielectric mirror, or by integrated optical components such as a Mach-Zehnder interferometer.
The 1.3 xcexcm waveguide was then separated into two ports consisting of an input and an output, either by a conventional Y junction or by a 3 dB coupler.
Such a transmission method nevertheless has many drawbacks.
This is because the separation between the input and the output of the 1.3 xcexcm wave routinely introduces a loss of 3 dB.
In addition, a xe2x80x9cping-pongxe2x80x9d effect is introduced into the voice communication channel because the uplink and downlink transmissions use the same 1.3 xcexcm window.
In addition, transmission on the 1.3 xcexcm channel is of low throughput, typically a few tens of megabytes per second.
The present invention thus seeks to produce a bidirectional multiplexer/demultiplexer with three wavelengths which allows on the one hand simultaneous bidirectional communication on two different wavelengths and on the other hand distribution on another wavelength.
In the context of an application to direct-access networks, the present invention proposes to use two different wavelengths for a bidirectional communication in the same 1.3 xcexcm window, for example 1.28 xcexcm and 1.32 xcexcm, and to separate them by means of an isotropic filter with a single passband in order not to interfere with the transmission of the optical wave at 1.5 xcexcm.
Thus another problem which the invention seeks to resolve is to manage to produce an optical filter making it possible to separate signals propagating at wavelengths close to each other (for example 1.28 xcexcm and 1.32 xcexcm).
French patent No. 2 732 478 describes a method of filtering two wavelengths by means of a codirectional coupler. Such a method is illustrated in FIG. 2.
This patent describes a structure with two optical waveguides having a bottom confinement layer 2, a core 3 for guiding the light and two strips 4 and 5 for loading the core and forming the optical guides. Such a structure is suitable for producing a filter, a coupling grating being etched on one of the strips 5.
With such a filter, when the light propagates in a waveguide, all the wavelengths except that of the filter pass through the guide in the direct channel, whilst the chosen wavelength is transferred into the lateral channel in the parallel coupled waveguide.
According to a particularity of the invention described in this patent, the thicknesses of the core and strips are defined so that the two optical guides have the same modal birefringence. The core and strips have in fact a succession of alternating thin layers respectively made from binary material and quaternary material.
Thus a codirectional asymmetric coupler makes it possible to separate a given wavelength xcex0, fixed by the periodic interference xcex9 etched on one of the strips, the other wavelengths propagating in the other waveguide whatever the state of polarisation of the signals.
The present invention seeks to produce an optical filter which makes it possible to extract a given wavelength xcex0, the coupler constituting this filter having a spectral response with a high rejection ratio and a narrow passband. This is because the efficiency of an optical filter is generally limited by the size of the secondary lobes of its transfer function.
FIGS. 3a and 3b illustrate the spectral responses respectively on the lateral channel and on the direct channel of a conventional optical filter consisting of an optical coupler such as the one described with reference to FIG. 2. It can be seen that the xcex0 passband xcex94xcex is relatively broad and that the rejection ratio xcfx84 is low. Such a coupler can therefore not be used for filtering a signal at a given wavelength xcex0 propagating with other signals at close wavelengths.
Different solutions have been proposed in the prior art for eliminating or reducing the secondary lobes of the spectral response of an optical filter. Such an operation is known as xe2x80x9capodisationxe2x80x9d of the filter transfer function.
One particular solution consists in effecting a progressive evanescent coupling. For example, it is possible to achieve a coupling profile like the one illustrated in FIG. 4a by producing curved waveguides (the profile of FIG. 4a is known as a generalised cosine profile). Such a coupling profile k(z) consists in varying the distance separating the waveguides of the coupler (along the Z-axis) over the entire length L of the filter, the coupling being at its maximum at the centre of the filter.
FIG. 4b depicts the transfer function obtained by such a progressive coupler. It will be noted that it has been possible to achieve a rejection ratio xcfx84 of approximately 30 dB. On the other hand, the width of the principal lobe xcex94xcex has been increased, which is a disadvantage in the case of an optical filter which has to separate wavelengths close to each other.
It is possible to envisage coupling profiles different from the one illustrated in FIG. 4a, but this makes the design of the filter even more complex, or introduces other disadvantages, such as a length of filter multiplied by two for a so-called xe2x80x9cbox-likexe2x80x9d coupling profile for example procuring a square shape.
For this purpose a quality coefficient Q is introduced, which represents the ratio of the width of the principal lobe at xe2x88x923 dB to its width at xe2x88x9220 dB: Q=xcex94xcexxe2x88x923dB/xcex94xcexxe2x88x9220dB 
FIGS. 5a and 5b illustrate simulated optical filter spectral responses for respective quality coefficients Q of 40% and 100%.
The present invention therefore seeks to produce an optical filter whose spectral response most closely approaches the ideal apodisation function (the function in which the secondary lobes have disappeared and where the principal lobe is narrow).
The first object of the present invention is therefore to achieve a multiplexing and/or demultiplexing function over at least three wavelengths in a single step.
The second objective of the present invention is also to achieve a function of optical filtering of at least one wavelength by means of a first grating-assisted codirectional asymmetric coupler to which there is added at least one other coupler, referred to as a dummy coupler, grating-assisted so as to increase the rejection ratio and reduce the width of the passband of the spectral response of the first coupler of the filter.
In particular, the present invention proposes a structure with three optical waveguides formed by three codirectional strips providing a bidirectional evanescent coupling assisted by at least one grating and dependent on the wavelength.
The object of the present invention is more particularly an optical multiplexer/demultiplexer able to combine and/or separate at least two optical signals amongst n propagating at different wavelengths, characterised in that it comprises at least one central waveguide and two lateral waveguides, each lateral waveguide constituting with the central guide a pair of waveguides, each pair being disposed so as to allow a bidirectional evanescent coupling of an associated wavelength between the guides in each pair, the coupling being selective with respect to wavelength and assisted by at least one etched grating, the said waveguides being designed so that the multiplexer/demultiplexer has a functioning independent of the state of polarisation of the signals.
According to a first embodiment, the central waveguide is etched with a coupling grating, the lateral waveguides being asymmetric so as to couple respectively a first and second wavelengths.
According to a second embodiment, each lateral waveguide is respectively etched with a first and second coupling grating so as to couple respectively a first and second wavelengths.
According to a third embodiment, the coupling gratings of each lateral waveguide are identical, the lateral waveguides being asymmetric so as to couple respectively a first and second wavelengths.
According to an essential characteristic of the present invention, each waveguide has the same modal birefringence.
According to a particularity of the present invention, the two coupled wavelengths propagate in opposite directions, the first being combined with the first lateral guide in the central guide when the second is separate from the wavelengths propagating in the central guide in order to be coupled in the second lateral guide, and vice-versa.
According to a particular embodiment, the two wavelengths coupled respectively by each lateral guide are situated in the same optical transmission window.
According to another characteristic, each lateral guide has a weighted interference in addition to the etching of a coupling grating so that the rejection ratio of the spectral response of each coupler of the multiplexer/demultiplexer is greater than or equal to 10 dB.
Advantageously, the weighted interference consists of a curvature of the lateral guides, with respect to the straight central guide.
Preferentially, the distance between the central guide and each lateral guide varies between 2 and 5 xcexcm.
According to a particular embodiment of the invention, the optical multiplexer/demultiplexer comprises a central waveguide and a plurality of pairs of lateral waveguides, each pair of lateral waveguides being able to successively couple two wavelengths (xcex1, xcex2).
Another object of the invention is an optical filter comprising at least one multiplexer/demultiplexer according to the invention.
According to one characteristic of this optical filter, a pair of waveguides constitutes a first coupler able to couple the wavelength of the signal to be filtered, the other pair or pairs of waveguides constituting dummy couplers able to couple one or more rejection wave or waves, close to the filtered wavelength, so as to increase the rejection ratio and to decrease the width of the passband of the spectral response of the first coupler.
The invention also relates to an optical transmitter comprising a plurality of photodiodes and a plurality of photodetectors, and also comprising a multiplexer/demultiplexer according to the invention.
The invention particularly applies to direct access networks comprising an Optical Line Terminal and a plurality of Optical Network Units, optical fibres connecting the latter to the former, characterised in that each terminal comprises an optical transmitter according to the invention.
According to a particular embodiment of the invention, at least three optical signals are propagated between the OLT and each ONU, a first 1.5 xcexcm optical signal intended for video distribution, and two other optical signals at 1.3xe2x88x92 xcexcm and 1.3+ xcexcm intended for a bidirectional voice communication.
Advantageously, the optical signals at 1.3xe2x88x92 xcexcm and 1.3+ xcexcm are respectively coupled in the lateral waveguides, the 1.5 xcexcm optical signal propagating in the central waveguide.
Preferentially, the optical signals intended for bidirectional communication are fixed at 1.28 xcexcm and 1.32 xcexcm.
The optical module according to the invention has the advantage of being simple to produce, and in particular of using known manufacturing techniques.
Advantageously, the same multiplexer/demultiplexer according to the invention can be used in the optical line terminal OLT, and in the optical network unit ONU.
This is because the optical multiplexer/demultiplexer according to the invention can easily be integrated into a monolithic component with laser diodes and photodetectors. The same component can be placed in the OLT or the ONU, only the relative arrangement of the different elements being different.
The use of two wavelengths for voice transmission at 1.3 xcexcm improves the transmission rate by a factor of ten.
It should also be noted that such a coupling concept can easily be extended to other transmission windows.